Filters and filter chains

Abstract

An apparatus, comprising: an audio input for receiving an input audio signal; an tuning input for receiving a tuning signal; a filter chain comprising a plurality of filters for filtering the audio signal to produce a filtered input audio signal, the filter chain comprising: a first filter module operating at a first sampling rate; and a second filter module operating at a second sampling rate greater than the first sampling rate, wherein a phase response of the first filter module is dependent on the tuning input and wherein a magnitude response of the first filter module is substantially independent of the tuning input.

The invention claimed is: 
     
       1. An audio filter system comprising:
 an oversampled filter chain, the filter chain comprising a plurality of audio filter modules, wherein a first filter module of the plurality of audio filter modules running at a lowest sample rate of the chain is arranged to receive a tuning signal and to adjust the phase response of the first filter module based on the tuning signal such that pre-ringing and/or post-ringing of the filter chain is varied. 
 
     
     
       2. The audio filter system of  claim 1 , comprising:
 an audio input for receiving an input audio signal; and 
 a tuning input for receiving the tuning signal; 
 wherein the oversampled filter chain is configured to filter the audio signal to produce a filtered input audio signal, 
 wherein the first filter module is configured to operate at a first sampling rate, 
 wherein the plurality of audio filter modules comprises a second filter module operating at a second sampling rate greater than the first sampling rate, and 
 wherein a magnitude response of the first filter module is substantially independent of the tuning signal. 
 
     
     
       3. The audio filter system of  claim 1 , wherein, in response to a change in the tuning signal, the first filter module is configured to transition between having a linear phase response and having a minimum phase response. 
     
     
       4. The audio filter system of  claim 2 , wherein, in response to an increase in the tuning signal, the first filter module is configured to:
 increase the level of pre-ringing in the filtered input signal due to the filter chain; and 
 decrease the level of post-ringing in the filtered input signal due to the filter chain increases. 
 
     
     
       5. The audio filter system of  claim 1 , wherein a second filter module of the plurality of filter module comprises a decimation filter, wherein the first filter module is provided after the second filter module in the filter chain. 
     
     
       6. The audio filter system of  claim 1 , wherein a second filter module of the plurality of filter module comprises an interpolation filter, wherein the first filter module is provided before the second filter module in the filter chain. 
     
     
       7. The audio filter system of  claim 1 , wherein the first filter module comprises:
 a plurality of first filters for generating respective first filtered signals, the plurality of first filters each having a different phase response; and 
 a multiplexer configured to switch between respective first filtered signals for output based on the tuning signal. 
 
     
     
       8. The audio filter system of  claim 7 , wherein the first filter module comprises:
 a pre-filter for generating a pre-filtered signal, wherein the plurality of first filters are configured to generate the respective first filtered signals based on the pre-filtered signal. 
 
     
     
       9. The audio filter system of  claim 8 , wherein the pre-filter comprises a minimum phase filter or a linear phase filter. 
     
     
       10. The audio filter system of  claim 7 , wherein the plurality of first filters each comprise an all-pass filter having a different phase response. 
     
     
       11. The audio filter system of  claim 7 , wherein during switching between respective first filtered signals the multiplexer is configured to output a weighted combination of two or more of the respective first filtered signals. 
     
     
       12. The audio filter system of  claim 7 , wherein the multiplexer is configured to switch between respective first filtered signals for output during a zero-crossing point of the input signal or during a period in which the input signal is below a signal threshold. 
     
     
       13. The audio filter system of  claim 1 , wherein the first filter module comprises:
 a first filter for generating a first filtered signal; and 
 a second filter for filtering the first filtered signal to generate a second filtered signal, 
 wherein a response of the second filter is dependent on the tuning signal. 
 
     
     
       14. The audio filter system of  claim 13 , wherein the second filter is an all-pass filter or wherein the second filter is adjustable between minimum phase and linear phase based on the tuning signal. 
     
     
       15. The audio filter system of  claim 13 , wherein an impulse peak in a response of the second filter is substantially independent of the tuning signal, and wherein pre-ringing and post-ringing of the second filter vary in dependence on the tuning signal. 
     
     
       16. The audio filter system of  claim 13 , wherein the first filter is of the same order as the second filter. 
     
     
       17. The audio filter system of  claim 1 , further comprising:
 a tuning module configured to adapt the first filter module based on a characteristic of an audio system comprising the audio filter system. 
 
     
     
       18. The audio filter system of  claim 17 , wherein the characteristic of the audio system comprises one of:
 a type of audio output device comprised in the audio system; 
 a type of amplifier comprised in the audio system. 
 
     
     
       19. An integrated circuit comprising the audio filter system of  claim 1 , wherein the tuning input is coupled to one or more pins of the integrated circuit. 
     
     
       20. An audio playback device comprising the audio filter system of  claim 1 , wherein the audio playback device comprises a high fidelity (Hi-Fi) stereo system. 
     
     
       21. A digital to analog converter (DAC) system comprising the audio filter system of  claim 1 . 
     
     
       22. A filter module comprising:
 a first filter configured to receive an input signal and provide a first filtered signal, and 
 a second filter configured to receive the first filtered signal and provide a second filtered signal as an output signal, 
 
       wherein the first and second filters present a combined filter response for the filter module, the second filter arranged to receive a tuning signal and dynamically adjust a level of pre-ringing of the filter module based on the tuning signal, while maintaining a substantially constant magnitude response of the filter module. 
     
     
       23. A filter module comprising:
 a first filter configured to receive an input signal and provide a first filtered signal, 
 a plurality of second filters configured to receive the first filtered signal and provide a plurality of second filtered signals, the plurality of second filters having different phase responses, and 
 a multiplexer configured to receive the plurality of second filtered signals and provide an output signal selected from the plurality of second filtered signals, wherein the first filter and the selected second filter present a combined filter response for the filter module, 
 wherein the multiplexer is arranged to receive a tuning signal and to select one of the plurality of second filtered signals to be output based on the tuning signal.